1. Field of the Invention
The present invention relates to the use of indentation and ultrasound techniques to quantitatively assess the mechanical properties of soft tissues, and in particular of a soft tissues covering a musculoskeletal system of a body.
2. Description of Prior Art
The human musculoskeletal system is entirely covered by layers of soft tissues. At the body support interfaces, such as buttock tissues interfacing with a seat, residual limb tissues interacting with a prosthetic socket, and plantar tissues of a foot interacting with an in-sole/ground, significant loads are transmitted via skin to underlying tissues. Biomechanical assessment of skin and underlying soft tissues is relevant to the designs of respective body support interfaces. These issues are relevant to many clinical rehabilitation problems, such as the design of special cushions for a spinal cord injury, special orthotics for diabetic feet, and custom prosthetic sockets for amputees. Traditionally, biomechanical properties of limb soft tissues are evaluated by palpation. Such a subjective assessment requires substantial experience acquired through trial and error. The qualitative nature makes knowledge accumulation difficult and makes teaching-learning imprecise.
Among the various biomechanical testing protocols, indentation test is an effective and relatively simple way to make biomechanical assessment of the skin and subcutaneous tissues under compression. Tests themselves very much resemble that of palpation. However, existing indentation apparatuses that have been proposed are not feasible for extensive clinical application. In most cases, indentors are driven by electromechanical or pneumatic devices. This makes testing systems difficult to handle, and difficult to operate, with potential hazards to the soft tissues. One of the latest example of electromechanical driving indentation apparatus was introduced in A. P. Pathak, et al., A rate-controlled indentor for in vivo analysis of residual limb tissues, IEEE Transactions on Rehabilitation Engineering; Vol. 6, 12-20 (1998). The application of this device in clinical field is still difficult due to the large dimension of the indentation device. A hand-held indentation apparatus with a laser distance sensor to monitor the indentor displacement is disclosed in M. Horikawa, et al., Non-invasive measurement method for hardness in muscular tissues, Medical and Biological Engineering and Computing; Vol. 31, 623-627 (1993). The measurement of the displacement by laser has the drawback that the result is significantly influenced by the misalignment, the indented area if the laser beam is too close to the indentor, and the curvature of the limb if the laser beam is too far from the indentor. Furthermore, the existing indentation apparatuses have the drawback that the deformation of the soft tissue is commonly determined by the movement of the indentor, so the soft tissue thickness can not be suitably measured in the test. The stiffness measured in this manner would reflect not only the material properties of the tissues but also some geometric factors. To extract accurate material parameter, the tissue thickness might be obtained using other approaches, such as MRI, X-ray and ultrasound techniques.
Conventional ultrasonic imaging techniques have been used to study the elastic properties of a soft tissue by measuring the strain in the tissue subjected to a given stress. An approach has been attempted to determine the tissue elasticity, by applying a low frequency vibration to the tissue surface while measuring the amplitude and phase of the internal tissue vibration using ultrasound Doppler technique. See e.g. T. A. Krouskop, et al., A pulsed doppler ultrasonic system for making non-invasive measurement of mechanical properties of soft tissue, J. Rehab. Res. Dev. Vol. 24, 1-8 (1987); see also R. M. Lerner, et al., xe2x80x9cSonoelasticityxe2x80x9d images derived from ultrasound signals in mechanically vibrated tissues, Ultrasound in Med. and Biol. Vol. 16, No. 3, 231-239 (1990), K. J. Parker, et al., U.S. Pat. No. 5,099,848 (1992), and Y. Yamakoshi, et al., Ultrasonic imaging of internal vibration of soft tissue under forced vibration, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 7, No. 2, 45-53 (1990).
Another method for imaging the tissue elasticity is disclosed in J. Ophir, et al., U.S. Pat. No. 5,107,837 (1992), J. Ophir, et al., U.S. Pat. No. 5,293,870 (1994), and M. O""Donnell, et al., Internal displacement and strain imaging using ultrasonic speckle tracking, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 41, 314-325 (1994). This method included emitting ultrasonic waves along a path into the soft tissue and recording an echo train resulting from an ultrasonic wave pulse. Another echo train was recorded resulting from a second ultrasonic wave emitting along the path after the tissue was compressed. A selected echo segment of the echo sequence corresponded to a particular echo source within the tissue along the beam axis of the transceiver. Time shifts in the echo segment are examined using cross-correlation technique to measure compressibilities of the tissue regions. The use of MRI for detection of shear waves in soft tissue induced by a mechanical actuator attached to the surface of the tissue is described in R. Muthupillai, et al., Magnetic resonance elastography by direct visualization of propagating acoustic strain waves, Science, Vol. 269, 1854-1857 (1995).
The above methods using ultrasound and MRI techniques are needed to generate the elasticity imaging of soft tissues. Their significance was the detection of tissue abnormalities, such as those caused by cancer or other lesions, and their output is the local mechanical property of internal regions of a soft tissue, not the bulk property of an entire tissue layer. For those applications, the deformation of the soft tissue was generally less than 5%, which was much smaller than those of the soft tissues interacting with supporting interfaces, such as residual limb tissue, for which 30% deformation is not uncommon in many cases. Since measurement of relative change of a material property is sufficient for many imaging purposes, these methods are usually not calibrated against any absolute values of the material parameters. This is a drawback when more accurate material parameters are needed.
It is an object of the invention to overcome these drawbacks.
According to the invention there is provided a portable ultrasonic palpation device for measuring Young""s modulus of a soft tissue layer comprising a hand holdable pressure applying probe having an ultrasonic transceiver for transmitting and receiving ultrasound at an outer surface of the tissue layer, an applied pressure sensor for the probe, and a programmed computer arranged to receive signals from the transceiver and from the pressure sensor and to compute the Young""s modulus of the tissue layer based on applications of manually applied different pressures to the outer surface.
The probe preferably comprises a cylindrical body having a forward tip containing the ultrasonic transceiver, and a like second cylindrical body attached end-to-end to the first cylindrical and containing the pressure sensor. The cylindrical bodies are preferably approximately 10 mm in diameter.
The computer may be programmed to provide output signals corresponding to an effective depth of the tissue layer.
The computer may be programmed to compute according to the equation:                     E        =                                            (                              1                -                                  v                  2                                            )                                      2              ⁢                              xe2x80x83                            ⁢              a              ⁢                              xe2x80x83                            ⁢              κ              ⁢                              xe2x80x83                            ⁢                              (                                  v                  ,                                      a                    /                    h                                                  )                                              ⁢                      P            w                                              (        1        )            
where E is the Young""s modulus, P the applied load, w the indentation depth, h the tissue thickness, a the radius of indentor, and K a scaling factor.